CODEWITHSUNDEEP
rustserde
otolock
otolock

Reputation: 710

Writing to file with Append(false) doesn't work as expected

I am learning to program with Rust and decided to build a CLI to manage my personal library. I'm still working on a quick proof of concept before going further so I have the barebones of what I need to work.

I am saving data to a file called "books.json" using std::fs and serde_json. The program works great the first time I run it, but on the second run, instead of overwriting the file, it is appending the data (for test purposes, it would add the same book twice).

Here's the code I have written so far. By using OpenOptions.append(false), shouldn't the file be overwritten when I write to it? 

use serde::{Deserialize, Serialize};
use serde_json::Error;
use std::fs;
use std::fs::File;
use std::io::Read;
use std::io::Write;

#[derive(Serialize, Deserialize)]
struct Book {
    title: String,
    author: String,
    isbn: String,
    pub_year: usize,
}

fn main() -> Result<(), serde_json::Error> {
    let mut file = fs::OpenOptions::new()
        .read(true)
        .write(true)
        .append(false)
        .create(true)
        .open("books.json")
        .expect("Unable to open");
    let mut data = String::new();
    file.read_to_string(&mut data);

    let mut bookshelf: Vec<Book> = Vec::new();
    if file.metadata().unwrap().len() != 0 {
        bookshelf = serde_json::from_str(&data)?;
    }

    let book = Book {
        title: "The Institute".to_string(),
        author: "Stephen King".to_string(),
        isbn: "9781982110567".to_string(),
        pub_year: 2019,
    };

    bookshelf.push(book);

    let j: String = serde_json::to_string(&bookshelf)?;

    file.write_all(j.as_bytes()).expect("Unable to write data");

    Ok(())
}

books.json after running the program twice: 

[{"title":"The Institute","author":"Stephen King","isbn":"9781982110567","pub_year":2019}]
[{"title":"The Institute","author":"Stephen King","isbn":"9781982110567","pub_year":2019},
{"title":"The Institute","author":"Stephen King","isbn":"9781982110567","pub_year":2019}]%

Upvotes: 3

Views: 3252

Answers (1)

otolock
otolock

Reputation: 710

Members of the Rust Discord community pointed out that by using OpenOptions, the file pointer was ending up at the end of the file when I wrote to it. They suggested I use fs::read and fs::write, and that worked. I then added some code to handle cases where the file did not already exist.

The main() function would then need to look something like this: 

fn main() -> std::io::Result<()> {
    let f = File::open("books.json");

    let _ = match f {
        Ok(file) => file,
        Err(error) => match error.kind() {
            ErrorKind::NotFound => match File::create("books.json") {
                Ok(fc) => fc,
                Err(e) => panic!("Problem creating the file: {:?}", e),
            },
        },
    };

    let data = fs::read_to_string("books.json").expect("Unable to read file");

    let mut bookshelf: Vec<Book> = Vec::new();
    if fs::metadata("books.json").unwrap().len() != 0 {
        bookshelf = serde_json::from_str(&data)?;
    }

    let book = Book {
        title: "The Institute".to_string(),
        author: "Stephen King".to_string(),
        isbn: "9781982110567".to_string(),
        pub_year: 2019,
    };

    bookshelf.push(book);

    let json: String = serde_json::to_string(&bookshelf)?;

    fs::write("books.json", &json).expect("Unable to write file");

    println!("{}", &json);

    Ok(())
}

Upvotes: 3

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